This invention relates to an arrangement for setting the impedance of a transmission line. In particular, it relates to an arrangement for setting the impedance of a circuit trace on a substrate such as a printed circuit board by introducing apertures in a conductive reference plane inside the board.
The impedance of a transmission line in the form of a conductive trace on a printed circuit board is determined by several factors. FIG. 1(a) is a schematic representation of a typical transmission line system. It includes a driver 10 with a characteristic impedance Zd, a transmission line 20 having a characteristic impedance Z1, and a receiving element or load 30 having a characteristic impedance Zr. An equivalent circuit for the line 20 itself is shown in FIG. 1(b). It includes a distributed series of circuits each made up of a characteristic inductance L, a characteristic ohmic resistance R, and a characteristic capacitive coupling C to other conductive bodies. At low frequencies, the impedance is dominated by the contribution of the ohmic resistance. At high frequencies, the reactance from the inductance and the capacitive coupling dominate the resistive component.
The conductive bodies of primary interest in connection with the capacitance of conductive traces on a printed circuit board are conductive reference planes such as a ground plane or a power plane located inside the board. This is illustrated in FIG. 2. FIG. 2 shows a substrate 40 having on it a first trace 50 and a second trace 60. A conductive reference plane 70 is located within the substrate 40. The capacitors shown in phantom indicate capacitive coupling between the traces 50 and 60 and the plane 70. The amount of capacitive coupling between the traces 50 and 60 and the plane 70 depends in general on the distance between the trace and the plane. A representative expression for the impedance of a PCB transmission line is: ##EQU1## where: T is the trace thickness
W is the trace width PA1 e.sub.r is the relative dielectric constant of the substrate PA1 H is the height of the trace above the reference plane. PA1 For conventional traces, the trace and the plane are about 5 mils (0.005 inches) apart. This arrangement typically results in impedances on the order of 50 ohms.
One or more additional planes, represented by the plane 80, may also be present inside the board, but they are effectively shielded from the traces 50 and 60 by the plane 70 and so have no significant effect on the impedance of the traces 50 and 60.
Because the impedance is dictated by geometry and material choices, and because these parameters are in turn usually determined by other considerations, trace impedance has been taken as a given, and little effort has been made to control trace impedance other than to avoid discontinuities. In most instances, a specific board will have characteristic values for each of the dimensions and parameters discussed above and most traces on the board will have generally constant impedances per unit length. There are circumstances, however, where it is desirable to have a trace impedance which is other than that which occurs fortuitously. For example, the Small Computer System Interface (SCSI) protocol calls for transmission lines having an impedance of 102 to 110 ohms. However, using typical board specifications, various dimensions or materials would have to be modified to achieve such an impedance using conventional design methods. There is thus a need for a way to set the impedance of a transmission line without undue compromise of the other circuit design criteria.